1. Field of the Invention
The present invention relates to an optical device, a control method of the optical device, and an image forming apparatus.
2. Description of the Related Art
In recent years, a surface-emitting laser called a vertical cavity surface emitting laser (VCSEL) capable of emitting several tens of (for example, about 40) laser beams from a single element is commercially available. Further, an image forming apparatus employing the VCSEL as a laser for exposing a photoreceptor to form images more accurately and rapidly has been proposed.
Meanwhile, in order to mount the VCSEL on the image forming apparatus, it is necessary to prepare a mechanism for adjusting emission beams of the VCSEL to emit a desired light amount. Back beams are not emitted from the VCSEL. For this reason, there is already known a technique for extracting a part of the emission beams (front beams) in a direction different from the emission direction of the emission beams using optical splitting unit for splitting the light beams into two directions and using them as front monitor beams.
As an example of the optical splitting unit, a half mirror or an aperture mirror is used. The half mirror reflects a part of the incident light beams and transmits the remaining light beams. The aperture mirror has an opening for transmitting light beams, for example, in the center of the mirror for totally reflecting the incident light beams.
The light amount of the front monitor beams split by the optical splitting unit (hereinafter, referred to as a front monitor light amount) is measured using an optical receiver such as a photodiode, and is fed back to the light amount control unit of the VCSEL to control the light amount of the emission beams of the VCSEL.
However, in the technique of controlling the light amount using the front monitor beams in the related art, it is difficult to accurately adjust the entire area of the light amount range for controlling the light amount to a desired light amount.
This problem will be described in more detail with reference to FIG. 12. In the related art, the light amount control was performed such that a front monitor light amount for the emission beams generated when the light-emitting element emits a predetermined light amount was measured, and the front monitor light amount for a desired emission beam light amount is calculated based on a primary equation using the origin and a single point obtained by the predetermined light amount and the front monitor light amount. In the example of FIG. 12, the light amount of the laser beam source is controlled based on prediction along a straight line 600 of the primary equation obtained from the origin (0, 0) and the measurement point (P1, Vpd1), where Vpd1 denotes the output (front monitor light amount) of the photodiode when light beams are output from a laser beam source at an emission light amount P1. For example, in order to obtain the emission light amount P2, the laser beam source is controlled such that the photodiode has an output value Vpd2.
Here, in the VCSEL, the light beams are emitted from each light-emitting source with an extent (divergence angle) extending in horizontal and vertical directions. As the (horizontal and vertical) divergence angles increase, the light beams are emitted with a larger extent. The divergence angle is changed depending on the change of ambient temperature of the VCSEL or a magnitude of the light amount of the emitted light beams.
In a case where the aperture mirror is used optical splitting unit, a ratio between the amount of light passing through the opening of the aperture mirror and the amount of the reflected light is changed depending on change of the divergence angle so as to change a ratio between the amount of emitted light beams and the amount of the reflected front monitor light. That is, as the divergence angle increases, a proportion of the amount of reflected light increases, and thus the light amount of the front monitor light increases comparative to the light amount of the emission beam light. Therefore, an actual relation between the actual amount of the emitted light beams and the amount of the front monitor light is highly likely to have a function that is not expressed as a primary equation.
For example, assuming that the divergence angle increases as the emission light amount increases, the proportion of the amount of light reflected by the aperture mirror also increases as the emission light amount increases. For this reason, as illustrated in a curve 601 of FIG. 12, in the area where the light amount is larger than that of the measurement point (P1, Vpd1), the front monitor light amount becomes larger than that predicted from the primary equation. In the area where the light amount is smaller than that of the measurement point (P1, Vpd1), the amount of the front monitor light becomes smaller than the light amount predicted in the primary equation.
Therefore, in a case where the light amount is controlled by feeding back the front monitor light amount, if the light amount at a certain light amount range is controlled based on a relational expression including the primary equation obtained from the amount of light beams of the VCSEL at a single point and the amount of the front monitor light, light amount control accuracy is degraded as much as a deviation amount from an actual curved relation. That is, in prediction using the primary equation, the emission light amount P2 is obtained by controlling the front monitor light amount along the straight line 600 such that the photodiode PD has an output value Vpd2. However, in practice, if the front monitor light amount is controlled such that the photodiode PD has the output value Vpd2, only the emission light amount P3 smaller than the emission light amount P2 is obtained along the curve 601.
Such a problem may occur even when the light amount is controlled by feeding back the back beam light amount.
In order to address such a problem, Japanese Patent Application Laid-Open No. 2009-65064 discloses a technique in which an aperture is also provided between the optical splitting unit and the photodiode in an optical system of the front monitor beam, and the front monitor beam also passes through the opening of the aperture as in the front mirror beam to increase light amount control accuracy. According to Japanese Patent Application Laid-Open No. 2009-65064, even when the (horizontal and vertical) divergence angles are isotropically changed, the light amount ratio between the emitted light beam and the front monitor beam is maintained constant so that the light amount control accuracy improves.
However, in Japanese Patent Application Laid-Open No. 2009-65064 described above, there is a problem in that, in a case where horizontal and vertical components of the divergence angle are changed with a different ratio, as illustrated in FIG. 13, an actual relation between the emission light amount and the front monitor light amount is deviated to the upper or lower direction, relative to the relational expression (straight line 600) based on the primary equation obtained from a single measurement point and the origin as illustrated as curves 602A and 602B due to a change method thereof.
In addition, which one of the upper or lower direction an actual relation between the emission light amount and the front monitor light amount is deflected to, relative to the relation determined by the primary equation, is different depending on the characteristic of an individual light source. For this reason, there is also a problem in that degradation of the light amount control accuracy accompanied by change of the divergence angle is not addressed as described above.